Comparators are non-linear circuits that are generally used to detect the sign differences between two or more signals and have been used to resolve signals in a variety of applications, such as memory and analog-to-digital converters (ADCs). A property used to describe the behavior of a comparator is its “time constant,” which indicates dependency of the propagation delay (or “clock to Q delay”) on the amplitude of the inputs. Typically, with a smaller the magnitude in the input signal, there is a longer delay to resolve the values it output terminals. This relationship between can be expressed as follows:TPROP=max(tFIXED,tFIXED−τ*ln(|VIN|)),where TPROP is the propagation delay, tFIXED is a fixed comparator delay related to (for example) process variation, temperature, and voltage on supply rail, τ is a time constant, and |VIN| is the magnitude of input signal (which is typically a differential signal). Usually, equation (1) holds for signals on the order of 100 mV or less, and, once the difference is sufficiently large, the propagation delay TPROP saturates to the fixed comparator delay tFIXED.
The performance of a comparator is often specified in terms of its input voltage sensitivity and propagation delay. The input voltage sensitivity sets the minimum detectable difference in input voltage required by the comparator. This is often described or limited by the comparator input offset voltage, which moves the comparator input decision threshold away from a theoretically ideal point. Undesirable changes in input offset voltage are often due to manufacturing variations of the comparator. In high performance applications, it is desirable to minimize both the input offset voltage and propagation delay.
As indicated from equation (1) above, there is a logarithmic relationship between the magnitude of the input signal and the comparator propagation delay, which can be seen in FIG. 1. When the comparator is being used to detect small differences in an input signal, the propagation delay increases dramatically, and when manufacturing variation cause input offsets similar in magnitude to the signals being compared, there may be some circumstances (such as when the offset is greater than the signal magnitude) where variations can affect the polarity of the decision. For example, if the input signal has a magnitude of 10 mV, a ±15 mV input offset could result in incorrect comparator “decisions.” In other circumstances, even when the input signal is greater than the offset, the performance of the comparator can be dramatically altered since the effective input amplitude, including the offset, is different depending on the polarity of the offset and the polarity of the input signal. For example, for a comparator with a +9 mV input offset with an input signal to the comparator of 9.1 mV, the effective input including offset is 18.1 mV, and, if the input signal to the comparator is −9.1 mV, the effective input including offset is −0.1 mV. For this example, the comparator would resolve both of these conditions to the proper polarity, but the propagation time would be very different for the two input signals. Referring to back to FIG. 1, for example, an input magnitude of 18.1 mV resolves in approximately 28 ps, and an input magnitude of 0.1 mV would resolve in approximately 54 ps. Thus, a relatively small input signal offset can dramatically affect the performance of a comparator when resolving input signals having a small magnitude. Therefore, there is a need for a method and/or apparatus that compensates for input signal offset.
Some examples of conventional systems are: U.S. Pat. No. 6,177,901; and Yang et al., “A Time-Based Energy-Efficient Analog-to-Digital Converter,” IEEE J. of Solid-State Circuits, Vol. 40, No. 8, August 2005, pp. 159-1601.